<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        :root{
            --color:red;
            --number:100px;
            --number2:100;
            --size:50px;
        }
        #box{
            background: var(--color);
            width: var(--number);
            height: calc(var(--number2) * 1px);
            font-size: var(--size,100px);
        }
    </style>
</head>
<body>
    <div id="box">aaaaaa</div>
</body>
</html>